The present invention relates to a solid state switch.
Switches are used to route current from a source to a load. With a mechanical switch, a conductive element is placed between the source and the load terminals. Generally a force on the mechanical switch is opposed by a spring mechanism that keeps the conductive element off the load terminals when the switch is not asserted. Mechanical switches include inherent weaknesses such as wear of contact surfaces and bouncing of the switch in a switching operation which may cause arcing and switch delays. In addition, mechanical switches that are actuated with electromagnets require high current levels to move contactors.
Solid-state switches, such as transistor-based switches, generally avoid these weaknesses, since there is no wear, no bouncing and no arcing in solid-state switches. Additionally, solid-state switches do not require high power levels to turn the switches on and off. However, when transistors are used as switches, the switch may be stuck in an “on” state if the transistor fails, since transistors tend to fail in an asserted state. In an example in which a transistor switch is used to drive a solenoid of a starting motor, failure of the transistor may continue to crank the starting motor until a battery voltage drops below a predetermined level corresponding to a hold-in current, or the starting motor may be damaged by over-running the motor.